Cellulosic fiber composites, which are used as wood substitute materials, have been available for many years. These composites, which include particle board, medium density fiberboard (MDF), oriented strand board (OSB) and laminated veneer lumber (LVL) among others, typically consist of wood particles of various sizes (depending on the product being produced) which are bound together with a relatively small fraction (5% to 15% on a weight basis) of thermosetting resin such as urea-formaldehyde or methyl-diphenylisocyanate (MDI). In addition to wood particles, these products have been produced using cellulosic agricultural materials such as straws or fibers such as hemp, jute and kenaf. While these products have been used successfully in a large number of applications as substitutes for solid wood, their use has been typically limited to applications which do not expose them to moisture, since they can deteriorate very quickly in wet or even highly humid environments. As such, they have seen very limited use in outdoor applications.
In order to overcome the poor moisture resistance of thermosetting resin cellulosic fiber composites, much attention and effort has been focussed recently on developing cellulosic fiber composites which use thermoplastic resins as a binder or matrix. These thermoplastic resin cellulosic fiber composites typically contain from 40% to 80% (on a weight basis) of cellulosic material typically derived from waste wood or cellulosic agricultural byproducts. Because of the higher resin content and the moisture resistant nature of the thermoplastic resins used in the production of these products, most thermoplastic resin cellulosic fiber composites typically perform substantially better than thermosetting resin cellulosic fiber composites in wet environments, and as such have developed a niche in outdoor applications such as decking, fencing and siding, among others. In this application, it is these thermoplastic resin cellulosic fiber composites which are being referred to when the term composite is used.
While thermosetting resin cellulosic fiber composites are typically produced via compression molding based processes, the majority of thermoplastic resin cellulosic fiber composites are produced via extrusion processes similar to the processes used to produce sheet, profile, pipe and the like from thermoplastic resins containing little or no filler. Most recent developments in the extrusion of thermoplastic resin cellulosic fiber composites have focussed on profile extrusion processes in order to produce profiles which could substitute for standard lumber (i.e. 2×4's, 2×6's, 1×8's, etc.) or milled wood profiles such as various window and door components. However, due to the typically high cellulose fiber content (40 to 80 weight percent), the extrusion of composite profiles presents challenges which are not typical of other extrusion processes.
The most significant issue in extruding composite profiles is the occurrence of melt fracture. In a paper titled “Profile Extrusion of Highly Filled Recycled HDPE” Suwanda, et al. describes melt fracture as a large scale deformation of the surface of the extrudate as it leaves the die resulting in tears in the extrudate and an unacceptably rough surface finish. Suwanda, et al. reported the occurrence of melt fracture for the extrusion of both solid and hollow composite profiles with the severity of the melt fracture increasing with decreasing die land temperature and increasing throughput. U.S. Pat. Nos. 5,082,605, 5,088,910, 5,096,046 and 5,759,680 issued to Advanced Environmental Recycling Technologies, Inc. of Springdale, Ark., discuss the possibility of the surface of the extrudate tearing as it exits the extrusion die when extruding composite profiles. Similarly, U.S. Pat. Nos. 5,851,469 and 6,527,532 issued to the Trex Company, LLC, Winchester, Va., describe how shear stress that develops between the extrudate and the die wall causing tears and roughness in the finished surface of the composite profile. In U.S. Pat. No. 5,725,939, Nishibori observed that in a conventional sheet die, the large frictional resistance of the inner surfaces of the die causes the composite material being extruded not to flow smoothly causing the extrudate to be non-uniform and for cavities to form in the extrudate.
In U.S. Pat. Nos. 5,088,910 and 5,096,046, Goforth, et al. disclose a process for producing solid composite profiles from a composite material comprising recycled thermoplastic resins and cellulosic fiber derived from waste wood by extruding the composite material through a heated die to produce an extrudate with a cross-section corresponding to that of the desired profile. Goforth, et al. teach that in order to overcome the occurrence of melt fracture, the surface temperature of the extrudate must be sufficiently high at the point where it exits the extruder to create a uniform surface and that if the proper surface temperature is not maintained, the surface of the extrudate may tear as it exits the extruder die. Similarly, in U.S. Pat. Nos. 5,082,605 and 5,759,680, Brooks, et al. teach that increasing the surface temperature within the heated die will improve the surface finish of the extrudate when extruding composite profiles.
In U.S. Pat. Nos. 5,851,469 and 6,527,532, Muller, et al. discloses a process for producing solid composite profiles, which consists of extruding a molten mass of composite material through a heated converging die to produce a shaped extrudate, which is then fed through a low friction thermally insulating land section into a passage connected to the insulating land which contains a cooling medium, such as water, that cools the extrudate enough to form a substantially dimensionally stable outer shell around the profile. The purpose of the low friction land section is to reduce the drag on the composite material in the land section thereby improving the surface finish of the extrudate by preventing melt fracture while the cooling medium is meant to preserve the surface finish by quickly solidifying the surface layer of the profile.
In U.S. Pat. No. 6,210,616, Suwanda discloses an extrusion process for producing composite profiles, which includes extruding a composite material comprising thermoplastic resin and cellulosic fibers through a die at a temperature above the softening point of the resin to form an extrudate having a desired cross-sectional shape, passing the extrudate through a die land and then feeding the extrudate through a thermal barrier insert member to a cooled shaper that is maintained at a temperature of about at least 20° C. below the softening point of the resin in order to solidify the outer skin of the profile sufficiently to maintain the shape of the profile after it exits the cooled shaper. Suwanda teaches that, without the cooled shaper of his invention, the extrudate would melt fracture.
U.S. Pat. No. 5,725,939 to Nishibori describes a process for producing composite profiles in which a mixture of thermoplastic resin and cellulosic fibers is extruded through a die whose inner walls are covered or coated with a layer of material, such as a fluoro resin, that has an extremely small coefficient of friction. The purpose of the low friction layer is to reduce the forces exerted on the extrudate by the die surfaces in order to extrude the desired composite profile without generating a rough surface.
U.S. Pat. No. 5,516,472 to Laver discloses an extrusion process for producing composite profiles in which the thermoplastic resin and cellulosic fibers are first dry-blended and then melt blended in an extruder. In addition to the thermoplastic resin, Laver also includes from 2.5% up to 20% by weight of thermosetting resin in his preferred material formulation. The blended material is extruded through a die system comprising a transition die, a stranding die and a molding die, wherein the transition die preforms the blended material to a shape approaching that of the end product, the stranding die forms individual strands from the blended material, and the molding die compresses the individual strands into the desired profile. Laver claims that his die system configuration and formulation allows the blended material to be processed at lower temperatures than those typically used to combine cellulosic fibrous material with thermoplastic resin, and because of the low temperatures, the die system configuration, and the individual strands used to form the final shape, traditional flow problems associated with solid part extrusion are eliminated.
While the above approaches to reducing the occurrence of melt fracture during the extrusion of composite profiles have been used with varying degrees of success to produce composite profiles with improved surface finishes, these approaches also have their drawbacks. Increasing the temperature of the die or the surface of the extrudate as is taught by Goforth, et al. and Brooks, et al. can result in degradation or burning of the cellulosic fibers if the die temperature is sufficiently high, thereby creating a burned appearance on the surface of the composite profile. Using a low friction land section to reduce the drag on the surface of the profile in the land section in combination with a passage connected to the land which contains a cooling medium as is taught by Muller, et al. is not easily adapted for the production of hollow articles. In addition, the abrasive nature of thermoplastic resin cellulosic fiber composite could cause substantial wear of the low friction land thereby changing the cross-section of the extruded profile with increasing wear. Using a cooled shaper to solidify the surface of the profile in order to prevent melt fracture as is taught by Suwanda has a substantial disadvantage, particularly with hollow profiles, in that once the surface of the profile is solidified, the ability to use the various methods known in the art to calibrate or maintain the desired cross-section of the extruded profile during cooling can be significantly limited. Nishibori's use of a die with inner walls that are covered or coated with a layer of material, such as a fluoro resin, that has an extremely small coefficient of friction has a disadvantage similar to Muller, et al. in that the abrasive nature of composite material could cause substantial wear of the low friction die wall material, thereby changing the cross-section of the extruded profile over time. Finally, the stranding die taught by Laver results in a composite profile which is not homogeneous in that there are no cellulosic fibers bridging the interfaces between the individual strands. This results in composite profiles which can easily crack between the strands when fasteners such as nails or screws are driven through the profile.
Therefore it would be desirable to have an improved die assembly for the extrusion of thermoplastic resin cellulosic fiber composite profiles meant to minimize and substantially prevent the occurrence of melt fracture on the surface of the extruded profiles. Preferably, the occurrence of melt fracture would be minimized and substantially prevented without the need for die temperatures which could possibly cause burning of the composite material or the need of low friction surfaces within the die assembly which can wear and thereby change the cross-section of the extruded profile. It is also desirable that the die assembly would be adapted to produce both solid and hollow composite profiles without solidifying the profile in the die assembly prior to calibration of the profile. Additionally, it is desirable to produce composite profiles without the need for a stranding die in the die assembly, so as to prevent cracking between the strands when fasteners such as nails or screws are driven through the profiles. It is also desirable to have processes which would incorporate the use of the improved die assembly to produce solid and hollow thermoplastic resin cellulosic fiber composite profiles.